Thermal barrier coatings (TBCs) are used to provide thermal protection for components in the hot gas flow of turbine engines. In addition to low thermal conductivity, these coatings require compliance, meaning flexibility or other strain tolerance, in order to withstand stresses from cyclic thermal expansion, vibration, and particle impacts. TBCs require strong adherence to the substrate. They are commonly made of ceramic materials such as yttria stabilized zirconia (YSZ) due to the refractory properties of ceramics. However, ceramic coatings do not readily adhere to metal surfaces, so a bond coat of a material such as MCrAlY (M=metal, Cr=chromium, Al=aluminum, Y=yttrium) is commonly applied between a metal substrate and the TBC. MCrAlY resists oxidation at high temperatures, and is compatible with a metal superalloy substrate and a ceramic TBC.
The TBC may be applied at less than full density to reduce thermal conductivity. However, present TBCs can densify during service asymptotically toward full density. This is due to tight conformance of ceramic splats to each other, resulting in small between-the-splat (inter-splat) gaps, which can close by sintering during service. As the splat interfaces disappear; the TBC becomes rigid and loses its ability to resist strains that occur during thermal cycling. This leads to spalling. Unmitigated cracking occurs, which allows the hot working gas to reach the bond coat directly, reducing its life. Since the inter-splat gaps reduce thermal conductivity, as they close, conductivity increases.
Various means have been proposed to overcome this problem, including inclusion in the TBC of hollow ceramic spheres, columnar cracking of the TBC, and surface grooving to provide compliance by segmentation. However, the TBC material can still sinter over time, thus increasing its conductivity and reducing its resistance to spalling. Materials that delay phonon propagation, such as low k Gadolinium, can be used, but they are more expensive than yttria stabilized zirconia.